Coagulation factors V (FV) and factor VIII (FVIII) are both secreted glycoproteins that share pivotal roles in both hemostasis and thrombosis. Although many secreted proteins (referred to as cargo) are believed to require transport receptors for efficient ER-to-Golgi transport, only a limited number of such receptors have been described, mostly in yeast. Evidence for the existence of mammalian cargo receptors came unexpectedly from studies of the human genetic disorder combined deficiency of FV and FVIII, which identified mutations in LMAN1 and MCFD2 as the cause of the disorder. Combined deficiency of FV and FVIII is a rare bleeding disorder characterized by coordinate reduction of both FV and FVIII to the range of 5-30% of normal. LMAN1 and MCFD2 form a Ca2+-dependent protein complex in the ER-Golgi intermediate compartment that interacts with FV and FVIII, suggesting that the LMAN1-MCFD2 complex is a cargo receptor required for efficient transport of FV and FVIII from the ER to the Golgi. Mice deficient in LMAN1 model the human disorder but also exhibit a strain-specific perinatal lethality that is not explained by the reduced FV/FVIII levels, indicating additional functions of LMAN1. Using LMAN1 and MCFD2 deficient mice, proposed research will elucidate structure-function relationship of the LMAN1-MCFD2 receptor complex, and its requirement for the secretion of FV and FVIII and other potential cargo proteins in vivo. Proposed studies will not only answer fundamental questions regarding the mechanism of LMAN1-MCFD2 receptor-mediated secretion of FV and FVIII, but will also provide fundamental new insights into general mechanism of mammalian ER-to-Golgi protein transport. Genetic deficiency of FVIII results in hemophilia A, which affects ~1 in 5000 males. On the other hand, a gain-of-function mutation in FV (FV Leiden) and increased FVIII activity are major risk factors for venous thrombosis, which affects ~1:1,000 individuals in the US per year. The findings will have practical importance for improving FVIII expression and may expedite the eventual goal of somatic cell gene therapy for hemophilia A, as well as new approaches to limiting FV and FVIII production in prothrombotic states.